To understand the microscopic picture of mixture adsorption on the graphite surface, we presented in Figure 4 the local density distributions of methane and ethane at point A marked in Figure 3 and a snapshot is shown in the inset. The distributions show a major peak near the surface and a much smaller peak is observed in the second layer. Ethane clearly dominates adsorption (higher peak of the first layer) because of its higher affinity with graphite. On the other hand, methane dominates in the second layer, where the influence of the surface force is less significant and therefore the relative contribution of methane and ethane is affected by the bulk phase which is methane-rich.